I have an inch Century that I am planning to build on. I see several problems, one is with the threads for the gas nut and the other is the additional machining to get the bolt to close all the way. One other thing is that it is designed to take metrix mags. If I want to use inch, there is some more machining.

It is nothing I can not handle but I should not have to address this on a 100% receiver

I have had good luck with Entreprise, have bought 5 and built on 4 of them so far.
But only they and DSA make true inch receivers(both Australian pattern). The Centurys are metric Type-3 with some inch features.

I'm trying to get one through DSA, but they have had me on the waiting list for 4 months now. Every time I call the girl that answers the phone tells me that there running production on another product right now & they will start running my receivers right after that.??? I don't know what to do now!

Entreprise can convert your metric to take the folding charging handle, tabbed topcover, and inch mag, but the outside will still be metric.
I had heard Entreprise was working on another batch of L1A1 receivers too. I have bought 5 of theirs.

Yes I'm having mine done byEntreprise. I was able to get ahold of a new DSA metric receiver. I called both Entreprise & DSA and both told me they were not ready to start building Inch receivers yet. they still had plenty of Metric orders to fill